Recent developments have occurred in the field of very small switches having moving liquid metal-to-metal contacts and that are operated by an electrical impulse. That is, they are actually small latching relays that individually are SPST or SPDT, but which can be combined to form other switching topologies, such as DPDT. (Henceforth we shall, as is becoming customary, refer to such a switch as a Liquid Metal Micro Switch, or LIMMS.) With reference to FIGS. 1-4, we shall briefly sketch the general idea behind one class of these devices. Having done that, we shall advance to the topic that is most of interest to us, which is a technique for hermetically sealing such switches when they are fabricated on a substrate.
Refer now to FIG. 1A, which is a top sectional view of certain elements to be arranged within a cover block 1 of suitable material, such as glass. The cover block 1 has within it a closed-ended channel 7 in which there are two small movable distended droplets (12, 13) of a conductive liquid metal, such as mercury. The channel 7 is relatively small, and appears to the droplets of mercury to be a capillary, so that surface tension plays a large part in determining the behavior of the mercury. One of the droplets is long, and shorts across two adjacent electrical contacts extending into the channel, while the other droplet is short, touching only one electrical contact. There are also two cavities 5 and 6, within which are respective heaters 3 and 4, each of which is surrounded by a respective captive atmosphere (10, 11) of a suitable gas, such as N2. Cavity 5 is coupled to the channel 7 by a small passage 8, opening into the channel 7 at a location about one third or one fourth the length of the channel from its end. A similar passage 9 likewise connects cavity 6 to the opposite end of the channel. The idea is that a temperature rise from one of the heaters causes the gas surrounding that heater to expand, which splits and moves a portion of the long mercury droplet, forcing the detached portion to join the short droplet. This forms a complementary physical configuration (or mirror image), with the large droplet now at the other end of the channel. This, in turn, toggles which two of the three electrical contacts are shorted together. After the change the heater is allowed to cool, but surface tension keeps the mercury droplets in their new places until the other heater heats up and drives a portion of the new long droplet back the other way. Since all this is quite small, it can all happen rather quickly; say, on the order of a millisecond, or less. The small size also lends itself for use amongst controlled impedance transmission line structures that are part of circuit assemblies that operate well into the microwave region.
To continue, then, refer now to FIG. 1B, which is a sectional side view of FIG. 1A, taken through the middle of the heaters 3 and 4. New elements in this view are the bottom substrate 2, which may be of a suitable ceramic material, such as that commonly used in the manufacturing of hybrid circuits having thin film, thick film or silicon die components. A layer 14 of sealing adhesive bonds the cover block 1 to the substrate 2, which also makes the cavities 5 and 6, passages 8 and 9, and the channel 7, each moderately gas tight (and also mercury proof, as well!). Layer 14 may be of a material called CYTOP (a registered trademark of Asahi Glass Co., and available from Bellex International Corp., of Wilmington, Del.). Also newly visible are vias 15-18 which, besides being gas tight, pass through the substrate 2 to afford electrical connections to the ends of the heaters 3 and 4. So, by applying a voltage between vias 15 and 16, heater 3 can be made to become very hot very quickly. That in turn, causes the region of gas 10 to expand through passage 8 and begin to force long mercury droplet 12 to separate, as is shown in FIG. 2. At this time, and also before heater 3 begins to heat, long mercury droplet 12 physically bridges and electrically connects contact vias 19 and 20, after the fashion shown in FIG. 1C. Contact via 21 is at this time in physical and electrical contact with the small mercury droplet 13, but because of the gap between droplets 12 and 13, is not electrically connected to via 20.
Refer now to FIG. 3A, and observe that the separation into two parts of what used to be long mercury droplet 12 has been accomplished by the heated gas 10, and that the right-hand portion (and major part of) the separated mercury has joined what used to be smaller droplet 13. Now droplet 13 is the larger droplet, and droplet 12 is the smaller. Referring to FIG. 3B, note that it is now contact vias 20 and 21 that are physically bridged by the mercury, and thus electrically connected to each other, while contact via 19 is now electrically isolated.
The LIMMS technique described above has a number of interesting characteristics, some of which we shall mention in passing. They make good latching relays, since surface tension holds the mercury droplets in place. They operate in all attitudes, and are reasonably resistant to shock. Their power consumption is modest, and they are small (less than a tenth of an inch on a side and perhaps only twenty or thirty thousandths of an inch high). They have decent isolation, are reasonably fast with minimal contact bounce. There are versions where a piezo-electrical element accomplishes the volume change, rather than a heated and expanding gas. There also exist certain refinements that are sometimes thought useful, such as bulges or constrictions in the channel or the passages. Those interested in such refinements are referred to the Patent literature, as there is ongoing work in those areas. See, for example, U.S. Pat. No. 6,323,447 B1.
To sum up our brief survey of the starting point in LIMMS technology that is presently of interest to us, refer now to FIG. 4. There is shown an exploded view of a slightly different arrangement of the parts, although the operation is just as described in connection with FIGS. 1-3. In particular, note that in this arrangement the heaters (3, 4) and their cavities (5, 6) are each on opposite sides of the channel 7. Another new element to note in FIG. 4 is the presence of contact electrodes 22, 23 and 24. These are (preferably thin film) depositions of metal that are electrically connected to the vias (19, 20 and 21, respectively). They not only serve to ensure good ohmic contact with the droplets of liquid metal, but they are also regions for the liquid metal to wet against, which provides some hysteresis in the pressures required to move the droplets. This is needed to guarantee that the contraction caused by the cooling of the heated (and expanded) operating medium does not suck the droplet back toward where it just came from. The droplets of liquid metal are not shown in the figure.
If contact electrodes 22-24 are to be produced by a thin film process, then they will most likely need to be fabricated after any thick film layers of dielectric material are deposited on the substrate (as will occur in connection with many of the remaining figures). This order of operations is necessitated if the thick film materials to be deposited need high firing temperatures to become cured; those temperatures can easily be higher than what can be withstood by a layer of thin film metal. Also, if the layer of thin film metal is to depart from the surface of the substrate and climb the sides of a channel, then it might be helpful if the transition were not too abrupt.
Some of the issues that surround the construction of a LIMMS device are a suitable hermetic seal and the control of electrical impedance for the signal lines served by the device. Hermetic construction is important, not so much because of the presence of mercury that needs to be sealed in to prevent its escape (the amounts involved are quite small and fly underneath regulatory radar, so to speak), as to assist in obtaining operational reliability by sealing out potential contaminants. For instance, a skin of oxidized mercury on the droplet can interfere with both mechanical motion and good electrical contact. Unfortunately, the CYTOP adhesive is slightly permeable to gases such as oxygen, and over a long period of time the mercury will develop an oxidized surface. The further issue of electrical impedance is important because LIMMS are sufficiently small that they lend themselves for use in high frequency applications where controlled impedance transmission lines are common. These might be strip lines or co-planar transmission lines.
One method of providing a hermetic seal for one or more LIMMS devices fabricated upon a substrate is to apply an outer cover over the LIMMS and any other nearby circuitry of interest. The outer cover itself may be of metal, ceramic or glass, and is impervious to contaminants, provides a high degree of mechanical protection, and if of metal, also offers potential electrical shielding. Metallic outer covers are typically soldered in place, which requires a ring of metal deposited on the substrate and matching the perimeter of the cover. This prevents any of the signal leads from traversing under the cover while on the same side of the substrate, and leads to the use of vias to get signals onto the other side of the substrate. Such use of vias might not be possible, or if it is, might not be convenient, either for reasons of cost or because of the detrimental effects of vias on controlled impedance RF conductors.
Glass and ceramic outer covers can be hermetically attached with glass frit, but the surface irregularities posed by same-surface signal lines can present potential difficulties, ranging from changes in surface height, issues of whether or not the surface of the signal line is readily wetted by frit, to imprecise electrical effects on the signal lines owing to uncontrolled variations in certain physical parameters. Attaching an outer cover directly to a substrate having top surface signal lines by using frit is not preferred, even though it might otherwise be desirable to use a glass or ceramic outer cover and attach it with frit.
In some applications it may be desirable to avoid the use of an outer cover plate, and leave the cover block of the LIMMS exposed. The use of CYTOP as an adhesive for the cover blocks is quite satisfactory, but it leaves something to be desired as a hermetic seal against the substrate. It is slightly permeable, and allows a slow oxidation of the mercury over the long term.
It would be desirable if there were a way to allow the use of a genuine hermetically sealed outer cover plate over the LIMMS devices without interfering with same-surface routing of signal traces to and from the LIMMS devices located beneath that outer cover plate, and to allow that hermetic seal without requiring the use of vias. Since one of the objections to the use of vias (aside from the possibility that the other surface might not be available for use!) is their ill effect on controlled impedance conductors, it follows that whatever technique allows an outer cover to cooperate with same-surface routing of signal conductors should likewise not produce undesirable impedance effects as those conductors pass under the perimeter of that outer cover. This remains so whether the outer cover plate is metallic and attached with solder or is non-metallic and attached with frit. It would also be desirable if, in instances where an outer cover plate is not desired or is inappropriate, there were still a good way to hermetically seal a LIMMS device cover block against the substrate while allowing the signal conductors to maintain same-surface routing and emerge from beneath the cover block without the use of vias. What to do?
A solution to the problem of obtaining an improved hermetic seal for one or more LIMMS devices on a substrate, and possibly having same-surface controlled impedance signal conductors on that substrate, is to either: (a) Enclose each entire LIMMS device beneath a common or respective outer cover that is separate from the LIMMS device(s), impervious to contaminants, and is hermetically sealed against the substrate; or (b) Fabricate each LIMMS device such that its individual cover block (which is already a component of the LIMMS and is not a separate outer cover or cover plate) can be hermetically sealed against the substrate. Each case must respect the presence of any same-surface signal conductors in the vicinity of the hermetic seal by limiting the effects of the hermetic seal upon impedances of those same-surface conductors.
In case (a) any same-surface conductors and the underlying substrate are covered with, or have affixed thereto, a layer of suitable dielectric material that is impermeable to contaminants and to the fluid and gas content of the LIMMS. That layer of dielectric material can essentially be the ribbon-like footprint of the perimeter of the separate outer cover, which may be recessed to accommodate the LIMMS device(s) it encloses. If there is to be no separate outer cover (case (b)), then the entire (solid) footprint of the LIMNS cover block on the substrate may receive a layer of the suitable and impermeable dielectric material deposited on or affixed to the substrate, which layer of such dielectric may itself then be covered, save for near its perimeter, with a layer of suitable adhesive. The perimeter footprint (in the case (a) of an outer cover) or the exposed perimeter (LIMMS cover block of case (b) and no outer cover) of the suitable and impermeable dielectric layer may be metalized. In case (a) the outer cover is soldered to the perimeter footprint (the outer cover may be metallic or if non-metallic, have a metalized region for receiving the solder). In case (b) a beveled edge of the cover block is also metalized, and the cover block is then soldered to the suitable dielectric layer subsequent to achieving adhesion with the layer of adhesive. In another embodiment for cases (a) and (b) glass frit is used in place of solder, and no metalized regions are required. In either of cases (a) and (b) the layer of suitable impermeable dielectric physically separates and insulates the various same-surface signal conductors from any conductive soldering.
This plan depends upon the use of a suitable dielectric material, which must be strong, adheres well to the substrate, is impervious to contaminants, is capable of being-patterned, and if also desired, which can be metalized for soldering. It should also have well controlled and suitable properties as a dielectric, so that would-be disturbances to signal line impedance can be consistently anticipated and appropriately compensated as those signal lines pass beneath structures presenting a change in capacitance (e.g., but not limited to, the conductive solder). Such compensation may include changes in signal conductor width in locations that pass beneath locations having solder. Given a choice, a lower dielectric constant (K) is preferable over a higher one. The layer of suitable dielectric material may be a thin sheet or gasket of previously patterned ceramic material, or it may be formed by the application of a thick film paste. Suitable thick film dielectric materials deposited as a paste and subsequently cured include the KQ 150 and KQ 115 thick film dielectrics from Heraeus and the 4141 A/D thick film compositions from DuPont.